As commonly defined, "replication" is something that happens in, and is managed by the database itself, or a daemon written for that purpose (whether the daemon works at the logical level, replicating database tuples, or at the physical level, replicating disk blocks), and of which an application can be largely, if not wholly agnostic. With two-phase commit, OTOH, an application has to be specifically written to take advantage of that functionality.

What happens if one of the nodes goes offline? System stops accepting requests or it keeps going and ends up in an inconsistent state?

Failure modes are defined by the two-phase commit protocol, the standard way of managing distributed database changes:

http://en.wikipedia.org/wiki/Two-phase_commit_protocol